The present invention relates to EDM apparatus and methods in general, and more particularly to an apparatus and method for monitoring the machining conditions during electrical discharge machining of an electrode workpiece by means of an electrode wire, in the course of which the electrical resistivity of the machining fluid across the electrodes is measured during the time intervals between consecutive machining discharges.
It is known, in diesinking EDM technology, to measure the resistivity of the machining fluid in the machining zone between two consecutive electrical discharges, for the purpose of detecting an excessive concentration of solid particles in the machining zone which is generally an indication of the formation of a bridge between the workpiece and the electrode tool preceding an imminent short circuit.
Measuring the resistivity of the machining fluid in the machining zone has also been used in travelling wire EDM technology, but for another purpose, mainly for measuring the thickness of the workpiece. The resistivity of the machining fluid is proportional to the thickness of the workpiece, which is an important parameter for achieving machining at optimum efficiency. Such a method of measuring the thickness of the workpiece is practical only under ideal machining conditions, in the course of which the specific resistivity of the machining fluid remains substantially constant.
It has been discovered that at the moment at which machining by an electrode wire deteriorates, the resistivity of the fluid increases between consecutive discharges, instead of decreasing as is the case during an EDM diesinking operation. This new and unexpected phenomenon, which is particular to travelling wire EDM machining, may be due to the sudden apparition of gas bubbles in the machining fluid, which results in entirely changing the conditions of heat exchange between the wire and the machining fluid, and which in turn causes rupture of the wire.